1. Field of the Invention
The invention relates to a vehicle planetary gear device.
2. Description of the Related Art
Generally, four-wheel drive vehicles are provided with a limited-slip differential (LSD)that has a planetary gear mechanism for a vehicle to lessen the rotation difference between the front wheels and the rear wheels if any rotation difference occurs. Typically, this type of vehicle planetary gear device is constructed of for example, as shown in FIG. 6A, a ring gear 31 that has a helical gear 31a on its inner peripheral surface; a sun gear 32 that has a helical gear 32a on its outer peripheral surface and that is disposed inside the ring gear 31 coaxially with the ring gear 31; a carrier 34 that is inserted between the ring gear 31 and the sun gear 32 coaxially with the ring gear 31 and the sun gear 32 and that has a hollow cylindrical portion provided with penetration holes 33 penetrating the hollow cylindrical portion along radial directions; and pinions 35 that are housed in the penetration holes 33 of the carrier 34 so as to be freely rotatable and be slidable in the axis direction and that has a helical gear 35a on its outer peripheral surface to mesh with the ring gear 31 and the sun gear 32.
In a planetary gear device as described above, when the front and rear wheels are rotating at the same rotation speed, that is, when there is no rotation difference between the front and rear wheels, the torque output from the engine is input to the carrier 34 via the input shaft 36. Then, with a predetermined torque distribution ratio, a certain torque is output to the front wheel side from the pinions 35 via the sun gear 32 while a certain torque is output to the rear wheel side from the pinions 35 via the ring gear 31. On the other hand, when a rotation difference occurs between the front and rear wheels, a rotation difference occurs between the sun gear 32 linked to the front wheel side and the ring gear 31 linked to the rear wheel side. In consequence, since the ring gear 31 and the pinions 35 are formed with helical gears, a thrust force shown by an arrow A in FIG. 6A is generated in the axis direction of the pinions 35. Due to this thrust force, the pinions 35 slide in the direction of the arrow A within the penetration holes 33 of the carrier 34, and thereby press the ring gear 31 since an end surface 35b of each pinion 35 contacts an inner wall surface 31b of the ring gear 31. Due to this pressing action, friction force on the contact portions between the end surfaces 35b of the pinions 35 and the inner wall surface 31b of the ring gear 31 is generated, and thereby the pinions 35 and the ring gear 31 are engaged. Therefore, the front wheel side linked to the pinions 35 via the sun gear 32 and the rear wheel side linked to the ring gear 31 become engaged. In consequence, another torque distribution that is different from the torque distribution occurring when the front and rear wheels are rotating at the same rotation speed is achieved such that the rotation difference between the front and rear wheels are lessened.
In this type of planetary gear device, when the rotation of the input shaft 36 is transmitted to the carrier 34, the pinions 35 provided in the penetration holes 33 of the carrier 34 revolve together with the carrier 34, and the pinions 35 press a inner wall of the carrier 34 since the outer peripheral surfaces of the pinions 35 contact inner wall surfaces 33a, 33b of the penetration holes 33. Due to this pressing action, concentrated stress, that is, so-called “Hertz stress”, is caused in the contact portions by the contact surface pressure. Due to the Hertz stress, friction is generated on the contact portions between the inner wall surfaces 33a, 33b of the penetration holes 33 and the outer peripheral surfaces of the pinions 35, and therefore the contact portions are likely to abrade.
In order lessen the abrasion of the contact portions, for example, Japanese Patent Application Publication No. 2003-314664 (hereinafter, JP-A-2003-314664) describes a vehicle planetary gear device in which the inner wall surfaces 33a, 33b of the penetration holes 33 are each formed by an arc plane whose curvature radius is same as the radius of the outer peripheral circle of each pinion 35. Therefore, the area of the contact portions at the time of contact between the outer peripheral surfaces of the pinions 35 and the inner wall surfaces 33a, 33b of the penetration holes 33 is enlarged, and therefore the contact surface pressure is decreased. Thus, the abrasion of the contact portions is reduced.
However, in the planetary gear device of JP-A-2003-314664, dimensions of the penetration holes 33, the pinions 35 and other component parts of the planetary gear device vary within the ranges of permissible dimensional error. In the case where such dimensional variations have accumulated when the inner wall surfaces 33a, 33b of the penetration holes 33 formed by arc planes whose curvature radius is same to the radius of the outer peripheral circle of each pinion 35, biased contact portions 38 may be formed between the inner wall surfaces 33a, 33b of the penetration holes 33 and the outer peripheral surfaces of the pinions 35 as shown in FIG. 6B. If such biased contact portions 38 are formed, the area of those contact portions lessens and the contact surface pressure heightens. In consequence, the abrasion of the contact portions becomes large. Incidentally, the “biased contact” refers to a type of contact by which the components parts whose evenly contact each other in the predetermined regions under a normal condition is caused to locally contact each other and thereby the surface pressure on the contact portions heightens due to accumulated dimensional errors of the component parts or the like.
Furthermore, biased contact portions 39 may be also formed between the end surfaces 35b of the pinions 35 and the inner wall surface 31b of the ring gear 31. If such a biased contact portion 39 is formed, the thrust force that occurs in the direction of the arrow B in FIG. 6B at the time of the differential control causes a moment force about the biased contact portion serving as a fulcrum to act in a direction orthogonal to the axis direction of the pinion 35. This moment force acts in the direction of an arrow D to press the outer peripheral surface of the pinion 35 against the inner wall surface of the penetration hole 33, and therefore further heightens the contact surface pressure on the biased contact portion 38, giving rise to problems of the abrasion of the contact portion becoming large and the production of noise from the contact portion being difficult to restrain.